Production of phosphoric acid by what is commonly known as the “wet process” involves the reaction of finely ground phosphate rock with sulfuric acid. As a result of the various reactions, a slurry is produced containing phosphoric acid, calcium sulfate and various impurities derived from the phosphate rock. This slurry is normally filtered to separate the phosphoric acid product from the byproduct calcium sulfate. The phosphoric acid thus obtained is then used in the production of various phosphate fertilizers such as, ammoniacal fertilizers via neutralization of the acid with ammonia.
Several varieties of this process are utilized around the World, but the most commonly employed is the Di-Hydrate process in which a specific crystalline form of calcium sulfate is produced by reaction of the calcium present in the raw phosphate ore, and the sulfuric acid used to acidulate the ore. Approximately 5 tons of calcium sulfate or gypsum are formed per ton of phosphate (P2O5) produced.
Water is normally used to wash the calcium sulfate filter cake and thereby increase the recovery of the phosphoric acid product. Most of this wash water is fed back into the phosphoric acid production process as make-up water. However, a portion of this water together with some residual phosphoric acid remains trapped in the calcium sulfate filter cake and is discharged from the filter with the filter cake. This trapped water contains several percent of phosphoric acid and small amounts of other impurities that were present in the raw material used to produce the phosphoric acid product. Additional water is normally used to help discharge the calcium sulfate filter cake off of the filter and then large volumes are used to transport it, by pumping as a slurry to a storage or disposal area. Thus, the discharge, or “pond water” as it is generally referred to, is created. The pond water generated in the process is of substantial volume and its storage and disposal are significant factors in the operation of a phosphoric acid plant. Measures to effectively address the by-products produced as embodied in the pond water are of great economic and environmental importance to the operators of the plant and the public.
At the storage or disposal area the calcium sulfate will settle and the excess transportation water will be liberated. This liberated water will normally be collected in a system of channels and ponds and recycled to the phosphoric acid production plant for reuse. The pond water is used for washing the calcium sulfate filter cake, for cooling and scrubbing process vapors, and can be used in grinding the rock to produce a slurry, and other purposes that do not require fresh water. These channels and ponds also serve as a collection site for other water that is used in and around the phosphoric acid plant, such as for cleaning or washing, fresh water fume scrubbers, and as a collection site for phosphoric acid spills or leaks within the plant. Also, since these channels and ponds are located outside, they collect rain water.
Since all of the water contained within these channels and ponds contains small amounts of phosphoric acid and other impurities normally present in the phosphoric acid, it is considered contaminated. Most particularly, the recycled or process water contains about 0.25% to 3% phosphoric acid, similar amounts of fluoride species, several hundred milligrams per liter of soluble ammonia, and trace amounts of many heavy and toxic metals. As such it is really not water in the traditional sense, but rather a weak but very acidic solution. The phosphoric acid plant pond water, or simply “pond water”, is also known as cooling pond water, gyp stack water, gypsum pond water, and wastewater. Pond water has many sources in a phosphate complex, barometric condenser water, scrubber water, and gypsum stack water. These streams report to a large open containment area referred to as the pond. A pond can cover many tens to hundreds of acres, and contain billions of gallons of acidic pond water. The accumulated water must be treated or purified to remove phosphoric acid and other impurities prior to being released to the environment. In some cases, in an efficiently operated phosphoric acid plant, in the absence of severe weather conditions, a balance will exist between water input to the pond system and water evaporation such that virtually all of this contaminated water can be recycled and used within the plant. In this case, treatment and discharge of the contaminated water, commonly known as pond water, is not necessary.
However, there are circumstances under which treatment and discharge of the contaminated pond water is necessary. One such circumstance could be an extended period of abnormally heavy rainfall. When the climatic cycle is such that rainfall and evaporation do not match, and one significantly outweighs the other, such as in the case of significant storm events or a drought, the process water balance is severely impacted. Of greatest consequence is the situation where several storms cause the collection and mixing of large quantities of storm water with the process water, raising of the water inventories to unsafe levels risking breach of the pond containment. Environmental damage can occur if the process water is abruptly discharged into local streams and rivers. Thus, it is necessary to routinely reduce excessive contaminated water (pond water) inventories to prevent its accidental discharge and the attendant environmental impact. Another scenario where it is necessary to address pond water inventory disposal would be when the phosphoric acid plant has ceased operation either for an extended period of time or permanently.
Many factors influence the specific components and their concentrations in this contaminated pond water. While it cannot be said that there is any typical composition for pond water some of the major chemical components that could be found in pond water, and an example of their range of concentrations, are as follows:
CHEMICAL COMPONENTRANGE OF CONCENTRATIONP1000-12,000ppmSO44300-9600ppmF50-15,000ppmSi30-4100ppm(ammoniacal) N40-1500ppmNa1200-2500ppmMg160-510ppmCa450-3500ppmK80-370ppmFe5-350ppmAl10-430ppmCl10-300ppm
Normally the major acidic components of pond water are phosphoric acid and sulfuric acid, with lesser amounts of hydrofluorosilicic acid (H2SiF6), and hydrofluoric acid (HF). The pond water is normally saturated or supersaturated with respect to many of its constituent ions, the only exception being after a period of heavy rainfall. Also, since the pond water is used for cooling it is continuously subjected to thermal cycling (heating and cooling). This thermal cycling, along with the addition of some waste fresh water to the pond water, is why the pond water can function as an effective scrubbing fluid for some process gasses.
The process of “double liming” has been the industry standard for treating or purifying pond water. A general schematic of the process as is known in the art is shown in FIG. 1. This method consists of adding a calcium compound (such as CaCO3, Ca(OH)2 or CaO) to the pond water, in two stages, such that the fluoride, phosphate and other impurities form solid precipitates that settle and are separated from the thus purified water. This method is described in Francis T. Nielsson, ed., Manual of Fertilizer Processing, Marcel Dekker, Inc. (1987), pp. 480 to 482; G. A. Mooney, et al., Removal of Fluoride and Phosphorus from Phosphoric Acid Wastes with Two Stage Line Treatment, Proceedings of the 33rd Industrial Waste Conference, Purdue Univ. (1978); G. A. Mooney et al., Laboratory and Pilot Treatment of Phosphoric Acid Wastewaters, presented at the Joint Meeting of Central Florida and Peninsular, Florida A.I.Ch.E. (1977); and U.S. Pat. Nos. 5,112,499; 4,698,163; 4,320,012; 4,171,342; 3,725,265 and 3,551,332.
FIG. 2 is a more detailed illustration of the conventional double liming process as shown in FIG. 1. During the first neutralization stage lime treatment 20a, lime 12 is added to pond water 10 to raise the pH of the solution to about 4 to 5, resulting in the precipitation of fluoride as CaF2 and/or CaSiF6. During this stage it is also thought that some of the hydrofluorosilicic acid present dissociates to HF and SiF4, with the SiF4 hydrolyzing to HF and SiO2. Some phosphate is also precipitated at this stage, as well as some calcium sulphate. The limed water, stream B, is then clarified 20b, being separated into a clarified overflow stream, stream D, and an underflow, stream E, containing the precipitated solids. The sludge produced at this stage is a granular, crystalline material that settles fairly rapidly and can be de-watered to about 30% solids in a gravity thickener. The sludge, stream E, can be sent to disposal 30 at the plant gypsum stack or recycled to the phosphoric acid plant for recovery of the phosphate.
In the final neutralization stage 60a, additional lime, is added to the clarified liquid from the first neutralization stage 20, stream D, to obtain a pH of about 8-10. In the final neutralization stage 60a, the remaining phosphates and fluorides are precipitated along with sulfate and many of the metals. The sludge in this stage has poor settling and thickening properties, due to the hydroxide nature of many of the compounds, and rarely achieves more than 5%-7% solids by weight. The sludge, stream H, from this final neutralization stage 60b is normally deposited 30b in large lagoons to allow for additional de-watering. In typical double liming, some of the final neutralization stage underflow, stream H, would be sent back to the pond water reservoir 10 or be added to the first stage 20a. However, without sufficient agitation, the bulk of the lime, coated with the reacted solids formed in the final stage 60a, would sit on the bottom of the pond. If too high, a proportion is added to the first stage, the system may become disrupted, due to the poor settling characteristics of these solids. For these reasons, most of the conventional double liming final stage underflow solids are impounded in their own pond 30b and the phosphate values lost.
If in this treatment, the final neutralization stage 60 clarified water, stream G, contains unacceptable levels of soluble ammonia, the final neutralization stage system 60a can be operated at a higher pH, from pH 10 to 12, to increase the un-ionized ammonia concentration, raising its volatility. The increased vapor pressure of the ammonia in solution facilitates its removal through air stripping 80 by the addition of spray devices located in or floating on the sedimentation lagoon. Even without the addition of a spray system 80, operation of the treatment lagoon at a pH of 9 to 10 or greater results in the removal of ammonia through volatilization due to the large surface area available for such activity.NH4++OH−=H2O+NH3 (Unionized and volatile)
The water stream G is treated with acid 80 to produce dischargeable water, stream I, that is discharged from the process 90.
The quantity of clear water that can be obtained from a double liming process is about 50%-70% of the feed volume. As an alternative, liming procedures can be carried out in a single stage.
However, there are several problems associated with this method. One problem is the large volume of sludge produced. Sludge (i.e., a mixture of the precipitated impurities, un-reacted calcium compounds and water) is produced in both the first and final stages of this process. These sludge materials, some of which de-water slowly, are normally deposited in settling ponds that require large land areas. Another significant problem with this treatment process is that very large volumes of lime are required to neutralize the acidic pond waters, some of which can have a pH as low as one.
It would be highly desirable to have a process that utilizes less lime in the treatment of the plant pond water to reduce the lime consumed in the treatment and, accordingly, the cost of treating the pond water. It would be highly beneficial to recapture a portion of the lime used in the process for reuse in the neutralization stages, thus limiting lime input. Furthermore, it would be highly desirable to have a method of capturing residual phosphate/phosphoric acid contained in the pond water thus enhancing overall phosphate production yields for the plant while simultaneously reducing the acidity and toxicity of the pond water, ultimately enabling its discharge to the environment. This invention serves these important needs and others as will become apparent.